An elongation of a steel material is generally reduced with an increase in strength. Recently, a hot stamping process in which a steel plate is austenitized by heating to a temperature of 900° C. or greater, followed by quenching for martensitic transformation has been applied in a variety of fields due to its advantages in both high strength and formability.
In such a hot stamping process, a steel plate is pressed and quenched simultaneously, using a special mold through which cooling water is circulated. Characterized by cooling parts fixed within the mold, the hot stamping process can precisely control dimensions of the parts, and does not cause a spring back phenomenon even after taking the plate out of the mold.
However, such hot stamping steel still has a corrosion problem. To solve the corrosion problem, the hot stamping steel is plated with Al—Si, which prevents the mother material from being directly exposed to a corrosive environment. However, as shown in FIG. 1, a Zn plating layer, even though damaged to expose the mother material, can still delay the corrosion of the mother material through active electron exchange with Fe of the mother material. In contrast, when the Al—Si plating layer is damaged, the exposed mother material may undergo rapid corrosion since Al and Si cannot exchange electrons with the mother material.
Accordingly, research has been conducted to overcome the insufficient corrosion resistance of the Al—Si-plated hot stamping steel plate. For example, it has been suggested that Zn may be applied to a hot stamping steel plate. Compared to conventional Al—Si-plated hot stamping, Zn-plated hot stamping has improved plating stability and can significantly improve corrosion resistance.
However, steel plates for use in vehicles have been plated with Zn whereas hot stamping steels are plated with Al—Si, because Zn is melted at the hot stamping process temperature of about 900° C. For example, at the hot stamping temperature of about 900° C., Zn may become unstable such that its practical application has been limited. Pure Zn melts at a temperature of 420° C. and vaporizes at a temperature of 907° C. while a Zn—Fe alloy has increased melting point with an increase in Fe content. The melting point of Fe—Zn changes with the composition thereof as shown in FIG. 2. Zn—Fe alloys produced in practical hot stamping process, however, may not have sufficiently high melting point because their Zn contents are generally of about 90 wt % or greater. Hence, only the components of Zn—Fe alloys cannot solve the problem of Zn melting. For Zn-plated hot stamping steel, the Zn of the plated layer is melted and then solidified during quenching in practical processes. In this procedure, numerous fine cracks are generated on the Zn surface due to the LME (Liquid Metal Embrittlement) phenomenon, significantly reducing corrosion resistance and plating stability.
Therefore, conventional methods may have difficulty in applying Zn plating to hot stamping. In particular, those techniques in the related art, in which a Zn layer is simply applied to hot stamping steel, may have problems such as generation of microcracks in the zinc plating layer, however, solutions to the problems have not been suggested.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.